(1) Field of the Invention
The present invention relates to a method for producing a thermoplastic polymeric sheet, more specifically to a method of extruding a molten thermoplastic polymer onto a moving quenching member where it is rapid-quenched and formed into a sheet, thereby producing a sheet of thermoplastic polymer.
(2) Description of the Prior Art
Various conventional methods are available for rapidly quenching a thermoplastic polymer and forming a sheet thereof utilizing static electricity. For instance there are U.S. Pat. Nos. 3,223,757 and 3,427,686 (hereinafter referred to as citation-1) which teach the method, as indicated in FIG. 1, in which a molten polymeric sheet 3 extruded out of an extrusion die 1 is brought into contact with a moving quenching member 2 to be quenched and solidified into a sheet. An electrode 5 is positioned on non-contact side of the molten polymeric sheet 3 near the point at which the sheet 3 contacts moving quenching member 2. Under the establishment of an electrical potential difference between the electrode 5 and the quenching member 2 through the use of a power supply 6, the sheet is produced. U.S. Pat. No. 4,310,294 (hereinafter referred to as citation-2) teaches a method wherein a polymeric sheet 3 is produced under the application of an electrical potential difference between the extrusion die 1 and the moving quenching member 2, as shown in FIG. 2. Japanese Patent Publication No. SHO 48-14784 (hereinafter referred to as citation-3) teaches the method shown in FIG. 3, wherein the moving quenching member 2 has an electrically insulated surface layer 2" on its surface 2'. The electrode 7 is set on the insulated surface layer 2" before it comes into contact with the molten polymeric sheet 3; and the sheet is produced under application of an electrical potential difference between the electrode 7 and the moving quenching member 2.
In all of these conventional methods, when the speed of the moving quenching member is increased to produce the polymeric sheet in a faster and more efficient manner, air is trapped between the sheet and the quenching member and as a consequence thereof the quality of the sheet surface tends to deteriorate. In citation-1, if the sheet is to be produced at a faster rate with no deterioration in the surface quality due to an entrapped air, either the electric potential of the electrode must be elevated or the electrode and the quenching member must be brought closer to each other. However, when the electric potential of the electrode is raised or the distance between the electrode and the quenching member is shortened, a arc discharge will be generated between the electrode and the quenching member, resulting in a substantial drop in the adherence of the sheet to the quenching member and a breakage of the quenching roller.
Thus in citation-1, it would be impossible to elevate the electric potential or to shorten the distance between the electrode and the quenching member to achieve faster production while at the same time achieving sufficient adherence. Accordingly 40-50 m/min. would be the limit of production speed. Furthermore, in citation-1, there are various problems, such as shortening of electrode life, breakage of the electrode and damage to the surface of the quenching member due to the corona discharge from the electrode to the quenching member which takes place when the polymeric sheet is defective (because the electrode is set with the polymeric sheet in between). These problems lead to various operational deficiencies.
Citation-2 and citation-3 are free from the above problems, but their limit in production speed is 25-30 m/min. which is considerably less than the value in citation-1.
The present inventors' investigations in trying to develop a method free from the above problems while achieving a higher limit of production speed revealed that in citation-2 the following problems occur at the limit speed of production:
(1) A very small discharge takes place between the moving quenching member and the polymeric sheet just before contacting the quenching member and as a consequence thereof the electrostatic adherence drops.
(2) When the electric potential is raised, an arc discharge takes place from the quenching member to the extrusion die from which the sheet is extruded and as a consequence thereof the adherence significantly drops and the surface of the quenching member is broken.
Thus citation-2 is also unsatisfactory in that the limit of production speed is low, because the electrical potential cannot be increased to the extent that production can be made faster while achieving sufficient adherence.